The present inventor proposed in a patent publication 1 a slide structure of a seat for a vehicle including movable and stationary rails, each of which is disposed to be along a longitudinal direction of the vehicle and includes a C-shaped cross-section.
The slide structure comprises a stationary rail which is fixed on a vehicle floor and extends in the longitudinal direction of the vehicle and a movable rail which fits over the stationary rail in the longitudinal direction so as to be slid on the stationary rail in the longitudinal direction, and the movable rail is connected to the cushion seat via a parallelogram link mechanism which functions to adjust the level of the cushion seat. The parallelogram link mechanism comprises front and rear links parallel to each other, the one end of each of which is pinned to the cushion seat and the other end of each of which is pinned to the side surface of the movable rail, so that each of the front and rear links can be rotated about the horizontal direction.
According to the above slide structure, in a case where a position of the cushion seat is to be adjusted, the longitudinal position of the cushion seat fixed to the movable rail can be adjusted by moving the movable rail relative to the stationary rail in the longitudinal direction in a sliding manner, while, on the other hand, the level of the cushion seat can be adjusted by moving the cushion seat relative to the movable rail in the vertical direction.
In particular, the longitudinal and vertical positions of the seat can be adjusted by a simple structure and the weight of the slide structure of the seat for the vehicle can be reduced, due to the fact that each of the movable and stationary rails includes a cross-section which is elongated in the vertical direction, in other words, it extends in the longitudinal direction and includes a side surface portion along the vertical direction.
However, such a slide structure raised the following technical problems, due to the fact that the shear load in the vertical direction is exerted on the movable rail via the front and rear links of the parallelogram link mechanism. That is, a smooth sliding function can be deteriorated in such a way that the movable and stationary rails can worsen to such a degree that they are brought into a locking situation, or a strange sound or a noise is generated upon a forcible sliding action, depending on the positional relationship between the movable and stationary rails.
More specifically, as shown in FIG. 23, in the slide structure of the seat for the vehicle, the downward shear load is exerted on the side surface of the movable rail 418 via the front link 460 and the rear link 462 of the parallelogram link mechanism, due to the weight of the passenger, or, as shown in FIG. 24, in case of a collision of the vehicle, the shear load is exerted on the side surface of the movable rail 418 via the front link 460 and the rear link 462 (upward direction at the front link 460, downward direction at the rear link 462).
On the other hand, in a case where the cushion seat is positioned to be in the most frontward position, the front end of the movable rail 418 protrudes forward from the front end of the stationary rail 416, while, on the other hand, in a case where the cushion seat is positioned to be in the most rearward position, the rear end of the movable rail 418 protrudes rearward from the rear end of the stationary rail 416. In either case, the movable rail 418 is brought into an overhanging situation.
Under such an overhanging situation, as described above, when the shear load is exerted on the side face of the movable rail 418 via the front link 460 and the rear link 462, as shown in an arrow in FIGS. 25 and 26, a torsion moment in which an arm length is defined to be from the shear center of the movable rail 418 to the point where the shear load is exerted on acts on the movable rail 418. In such a case, in accordance with a value of the torsion moment, the smooth slide function between both rails can deteriorate in such a way that the movable rail 418 can be locked against the stationary rail 416, or strange sounds can be generated when the movable rail 418 is forcibly slid relative to the stationary rail 416.
Here, the shear center is defined to be a point where an acting line of a resultant force of a shear stress on a cross-section (its value and its direction correspond to a shear force) never fails to pass through. In general, if a lateral load acting on a beam does not pass through the shear center, not only a deflection but also a torsion can be generated. In particular, resistance to torsion of an open cross-section is by far smaller than that of a closed cross-section, so that the torsion generated on the open cross-section becomes large.    Patent Publication 1: Patent Application No. 2008-297333